Organic compounds

ABSTRACT

The present invention relates to a novel selection system for use in a eukaryotic cell culture process and for expression of a recombinant product of interest. The selection system is based on the introduction of an exogenous functional membrane-bound folate receptor gene together with the polynucleotide or gene encoding the product of interest into a eukaryotic cell and can be widely utilized with eukaryotic cells for which cellular viability is dependent upon folic acid uptake.

This is a continuation of U.S. application Ser. No. 13/495,043 filed onJun. 13, 2012, which is a continuation of U.S. application Ser. No.12/808,704 filed on Jun. 17, 2010, which is a National Stage ofInternational Application No. PCT/EP2008/068046 filed on Dec. 19, 2008,which claims priority under 35 U.S.C. §119 to EP Application Serial No.07150326.2 filed Dec. 21, 2007, each of which applications in itsentirety is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a novel selection system for use in aeukaryotic cell culture process and for expression of a recombinantproduct of interest. The selection system is based on the introductionof an exogenous functional membrane-bound folate receptor gene togetherwith the polynucleotide or gene encoding the product of interest into aeukaryotic cell and can be widely utilized with eukaryotic cells forwhich cellular viability is dependent upon folic acid uptake.

BACKGROUND OF THE INVENTION

Selection markers and selection systems are widely used in geneticengineering, recombinant DNA technology and production of recombinantproducts, for example antibodies, hormones and nucleic acids, ineukaryotic cell culture. The primary goal of such dominant selectionmarkers and selection systems is to introduce a selectable gene whichupon exposure to selective growth conditions provides cells capable ofhigh-level production of the recombinant products of interest.

To date, there are 3 major selection marker systems available:

(a) The glutamine synthetase system: The enzyme glutamine synthetase(GS) is responsible for the biosynthesis of glutamine from glutamate andammonia. This biosynthetic reaction provides the sole pathway forglutamine formation in mammalian cells. Thus, in the absence ofglutamine in the growth medium, the enzyme GS is essential for thesurvival of mammalian cells in culture. Importantly, certain mammaliancell lines including mouse myeloma cells lack the expression ofsufficient GS and thus cannot survive without exogenously addedglutamine. Hence, such a cell line is an suitable acceptor for atransfected GS gene that in this system can function as a selectablemarker that allows for cell growth in a medium lacking glutamine. Incontrast, cell lines such as the widely used Chinese hamster ovary (CHO)cells express sufficient GS to support growth in glutamine-free medium.Therefore, if these CHO cells are to be used as the recipient cells forthe transfection of the GS gene, the specific and potent GS inhibitormethionine sulfoximine (MSX) can be applied in order to inhibitendogenous GS activity such that only transfectants expressing highlevels of the transfected GS gene can survive in a glutamine-freemedium. A major disadvantage of the GS system is the relatively longtime (i.e. 2-6 months) of selective growth in order to establish cellsstably overexpressing the target gene of interest. Another disadvantageis the frequent utilization of the cytotoxic agent MSX for theaugmentation of the selective pressure. The presence of such a cytotoxicagent along with a recombinant product of interest (e.g. a polypeptidelike an antibody) may require additional purification steps to rid ofthis cytotoxic agent.

(b) The dihydrofolate reductase/MTX selection system: Dihydrofolatereductase (DHFR) catalyzes the NADP-dependent reduction of dihydrofolicacid to tetrahydrofolic acid (THF). THF is then interconverted to10-formyl-THF and 5,10-methylene-THF which are used in the de novobiosynthesis of purines and thymidylate, respectively. DHF is thebyproduct of the catalytic activity of thymidylate synthase (TS) whichcatalyzes the conversion of dUMP to dTMP in a5,10-methylene-THF-dependent reaction. Thus, DHFR is crucial for therecycling of THF cofactors that are essential for the biosynthesis ofpurine and pyrimidine nucleotides that are necessary for DNAreplication. Hence, cells (e.g. CHO cells) that lack the DHFR gene (i.e.by targeted genomic deletion) can be used as recipients for thetransfection of the DHFR gene in a medium that is free of nucleotides.After transfection, the cells can be subjected to a gradual increase inthe concentrations of the antifolate MTX, a most potent DHFR inhibitor(Kd=1 pM), thereby forcing the cells to produce increased levels ofDHFR. Upon multiple rounds of selection, the selectable marker DHFRfrequently undergoes significant gene amplification. Furthermore, amutant mouse DHFR with a major resistance to MTX has also beenextensively used as a dominant selectable marker that markedly enhancesthe acquisition of high level MTX-resistance in transfectant cells. Amajor disadvantage of the DHFR/MTX selection system is that thistechnique utilizes a mutagenic cytotoxic agent, MTX, that can readilyalter the genotype of the recipient cells. Additionally, specific safetymeasures may have to be taken to protect the persons handling suchagents. This frequently results in MTX-resistant cell populations inwhich no expression of the target gene of interest is present due toloss of function mutations in the reduced folate carrier (RFC) and/orloss of RFC gene expression, both of which abolish MTX uptake. Anotherdisadvantage is that the mutagenic drug MTX may readily contaminate thesecreted overexpressed target product (e.g. a polypeptide like anantibody) contained in the growth medium thereby requiring laborintensive, time-consuming and expensive chromatographic methodsnecessary to rid off this mutagenic compound, MTX. In addition, theabsence of MTX in the final product has to be demonstrated by respectiveassays.

(c) The reduced folate carrier selection system: The reduced folatecarrier (RFC) is a ubiquitously expressed membrane glycoprotein thatserves as the major transporter for the uptake of reduced folates suchas 5-methyl-THF and 5-formyl-THF. However, RFC displays a very pooraffinity for the oxidized folate, folic acid. Hence, cells that lack theexpression of RFC or have been deleted for the genomic RFC locus canserve as recipients for the transfection of the selectable marker geneRFC under conditions in which reduced folates such 5-formyl-THF aregradually deprived from the growth medium thereby forcing the cells toexpress increased levels of the this folate transporter. There areseveral disadvantages for the RFC selection system: a) One must useRFC-null recipient cells in which the endogenous RFC locus has beenknocked out or inactivated by targeted knockout or loss of functionmutations. b) RFC has an extremely poor transport affinity for folicacid and thus this oxidized folate cannot be used for selection. c) Asopposed to the current folate-receptor based system that is aunidirectional folate uptake system and which will be explained indetail below, RFC is a bi-directional folate transporter that exhibitsequally potent import and export of folates. This implies that underconditions of folate deprivation, RFC overexpression may be detrimentalto the recipient cells that further export folate via the overexpressedRFC.

The aim of the present invention is to provide a novel metabolicselection system that has certain advantages over the prior artselection systems mentioned above. The novel selection system is basedupon the use of folates in the cell culture medium and on the presenceof folate receptors introduced via an expression vector into therecombinant eukaryotic cell intended to produce a product of interest.This novel approach requires no prior deletion of an endogenous folatereceptor (FR) gene. Following the introduction of a vector harboringboth the FR selectable gene as well as the polynucleotide encoding aproduct of interest (like a polypeptide), cells are grown in a selectivemedium containing highly limiting concentrations of folates. Hence, onlycells that markedly overexpress FR can take up sufficient folates tosustain cell growth, DNA replication and cellular proliferation, therebyallowing for overexpression of the target product of interest.

The oxidized folate, i.e. folic acid, as well as reduced derivatives offolic acid, known as reduced folates or tetrahydrofolates (THF) are agroup of B-9 vitamins that are essential cofactors and/or coenzymes forthe biosynthesis of purines, thymidylate and certain amino acids ineukaryotic, in particular mammalian, cells. THF cofactors areparticularly crucial for DNA replication and hence cellularproliferation. Specifically, THF cofactors function as donors ofone-carbon units in a series of interconnected metabolic pathwaysinvolving de novo biosynthesis of purines and thymidylate, amino acidsas well as methyl group metabolism, including CpG island methylation ofDNA. Specifically, THF cofactors including 10-formyl-THF (10-CHO-THF)contribute one-carbon units in two key de novo formyltransferasereactions involved in the de novo biosynthesis of purines. The firstenzyme, glycinamide ribonucleotide transformylase (GARTF), is involvedin the formation of the imidazole ring of purines, whereas the moredownstream reaction mediated by 5-aminoimidazole-4-carboxamideribonucleotide transformylase (AICARTF) yields the purine intermediateinosine 5′-monophosphate (IMP). The latter serves as a key precursor forthe regulated biosynthesis of AMP and GMP. Furthermore,5,10-methylene-THF (5,10-CH₂-THF), is another important THF coenzymewhich functions as a crucial cofactor for the enzyme thymidylatesynthase (TS). TS catalyzes the formation of thymidine monophosphate(dTMP) from dUMP. Hence, these folate-dependent enzymes are keymediators of the de novo biosynthesis of purine and thymine nucleotidesessential for DNA replication. As such, these folate-dependent enzymeswere identified as targets for the activity of folic acid antagonistsknown as antifolates. For example, the 4-amino folic acid analogueaminopterin and its homologue 4-amino-10-methylfolic acid, methotrexate(MTX) were the first class of antimetabolites that were introduced tothe clinic for the chemotherapeutic treatment of childhood acutelymphoblastic leukemia (ALL). Antifolates are currently key componentsof different chemotherapeutic regimens currently used for the treatmentof other human malignancies including osteosarcoma, breast cancer,primary central nervous system lymphoma, choriocarcinoma and gestationaltrophoblastic neoplasia.

In contrast to most prokaryotes, plants, fungi and certain protestswhich synthesize their own folates, mammals and other eukaryotic speciesare devoid of THF cofactor biosynthesis and must therefore obtain themfrom exogenous sources. Three independent transport systems arecurrently known to mediate the uptake of folates and antifolates inmammalian cells:

a) The predominant cellular transport system of reduced folate cofactorsis the reduced folate carrier (RFC). The RFC (also known as solutecarrier family 19 member 1, SLC19A1) is a ubiquitously expressed ˜85 kDamembrane glycoprotein functioning as a bi-directional facilitativecarrier that mediates the uphill transport of reduced folates byexchanging organic phosphates such as adenine nucleotides that are knownto accumulate to very high intracellular levels as well as thiaminemono- and pyrophosphate. RFC displays a high-affinity for THF cofactorsincluding leucovorin (5-formyl-THF; Kt=1 μM), while harboring only avery poor transport affinity (Kt=200-400 μM) for folic acid, an oxidizedfolate.

b) Another route of folate uptake is the proton-coupled folatetransporter (PCFT, also known as SLC46A) which has recently been cloned.PCFT appears to be expressed independently of the RFC, functionsoptimally at acidic pH (5.5) and mediates the influx of both oxidized(e.g. folic acid) and THF cofactors (i.e. reduced folates) as well asvarious hydrophilic antifolates including MTX. PCFT, which shows anoptimal transport of folates and antifolates at acidic pH (5.5) but noneat physiological pH (7.4), has a key role in the absorption of bothfolates and antifolates in the upper small intestine.

c) The third transport route, on which the present invention is based,involves folate receptors (FRs). FRs are high-affinity folate-bindingglycoproteins encoded by three distinct genes FRα (FR alpha), FRβ (FRbeta) and FRγ (FR gamma). FRα (or FR-alpha) is also known as AdultFolate Binding Protein or FDP, as Folate Receptor 1 or FOLR (in micefolbp1), and as Ovarian cancer-Associated Antigen or MOv 18. FRβ (or FRbeta) is also known as FOLR2 (fetal) and as FBP/PL-1(placenta). FRγ (orFR gamma) is also known as FOLR3 and as FR-G (reviewed by M. D. Salazarand M. Ratnam, Cancer Metastasis Rev. 2007 26(1), pp. 141-52.). Themature FRs, which are well-characterized, are homologous proteins with˜70-80% amino acid identity and contain 229 to 236 amino acids as wellas two to three N-glycosylation sites. FRα (FR alpha) and FRβ (FR beta)are membrane-bound, in particular glycosylphosphatidylinositol(GPI)-anchored, cell surface glycoproteins, whereas FRγ is devoid of aGPI anchor and is a secreted protein. FRα (FR alpha) and FRβ (FR beta)display a high affinity for folic acid (Kd=0.1-1 nM),5,10-dideazatetrahydrofolic acid (DDATHF; Iometrexol; Ki=0.4-1.3 nMusing [³H]folic acid as a substrate) and BGC945 (which is acyclopenta[g]quinazoline-based, thymidylate synthase inhibitorspecifically transported solely via FRα (FR alpha) and not via thereduced folate carrier) (Kd=1 nM), but much lower affinity for MTX(Kd>100 nM). FR-dependent uptake of folate and antifolates proceeds viaa classical mechanism of receptor-mediated endocytosis. Gene knockoutstudies have shown that FRα (FR alpha) (also known as Folbp1 in mice) isessential for early embryonic development and maternal folatesupplementation rescued from in utero embryonic lethality and allowedfor normal development.

There is an ongoing need for a safe, highly effective and cost-efficientselection system which overcomes one or more of the disadvantages of theselection systems known up to date.

SUMMARY OF THE INVENTION

The present invention relates to a eukaryotic expression vectorcomprising a first polynucleotide encoding a functional membrane-boundfolate receptor and a second polynucleotide encoding a product ofinterest.

The present invention further relates to eukaryotic cells for whichcellular viability is dependent on folic acid uptake, and into which thesaid expression vector has been stably introduced such that thefunctional folate receptor encoded by the vector is expressed by thecells.

Furthermore, the present invention relates to a selection method forproviding a recombinant eukaryotic cell capable of stably expressing theproduct of interest in high yields.

The present invention can favorably be utilized in a process forproduction of the product of interest in high yields.

DETAILED DESCRIPTION OF THE INVENTION

In the course of the present invention, it has now surprisingly beenfound that a selection system for providing recombinant eukaryotic cellscapable of producing a product of interest can be based on the limitedavailability of a folate in a cell culture medium. The system will bewidely applicable, i.e. to a eukaryotic cell which cellular viabilitydepends upon the uptake of a folate.

The novel system can be used for the accelerated selection, screeningand establishment of eukaryotic, for example mammalian, cell clones thatstably overexpress high levels of recombinant products in the absence ofcytotoxic drugs. Even more, and in contrast to other known selectionsystems, there is no essential need (although sometimes feasible) formodified cells, provided e.g. by mutating or knocking out endogenousgene(s). Since e.g. FRα (FR alpha) displays a higher affinity for FA(K_(D)=0.1 nM) than, for example, RFC for leucovorin (Kt=1 μM), andtransports folic acid into cells via a unidirectional pathway thepresent invention provides for the use of FRα (FR alpha) and otherfolate receptors as a markedly improved dominant metabolic selectablemarker, in particular, via gradual folate (e.g. folic acid) deprivationfrom the growth medium. The novel folate-based selection is an excellentstrategy that is well-suited for the accelerated, stable and high leveloverexpression of target proteins in cultured mammalian cells in theabsence of cytotoxic drug selection as routinely used in variousoverexpression systems.

The novel selection system shows several important advantages overselection systems available in the prior art.

1. The selection system according to the present invention is a veryrapid selection system: Within four weeks of folic acid deprivation,cell population or clonal cell derivatives expressing the target gene ofinterest can be readily isolated. This is in contradistinction to the GSsystem mentioned above which may require 2-6 months of selection andstabilization of the target gene.

2. The selection system according to the present invention does notrequire a genomic deletion or attenuation of the endogenous FRα (alpha),β (beta) or γ (gamma) genes prior to transfection and thus can beapplied to any recipient cell even when some endogenous FR geneexpression is present. This key advantage is based upon the fact thatfollowing FRα (FR alpha) transfection, cells can be exposed to an abruptand severe deprivation of folates (e.g. folic acid) from the growthmedium. Consequently, only transfectant cells which express significantamounts of the selectable FRα (FR alpha) marker can transport sufficientfolate to sustain DNA replication and cellular proliferation. Thisoccurs in the absence of any significant elevation in the expression ofthe endogenous FRα (FR alpha) gene. This is in contrast to the DHFR/MTXsystem mentioned above in which the recipient cells are frequentlydeleted for the endogenous DHFR gene (e.g. CHO DG44 cells and CHO Duxcells).

c) The selection system according to the present invention does notsuffer from the loss of stringency of selection due to alleviation ofthe selective pressure via increased expression of alternative routes offolate uptake including increased expression of the endogenous RFC. Thisimportant advantage is due to the fact that whereas FRα (FR alpha) hasan outstanding affinity for folic acid (Kd=0.1 nM), the RFC displays anextremely poor affinity for folic acid (Km=0.2-0.4 mM). In contrast,various prior art selection systems including the DHFR/MTX system cansuffer from a severe loss of stringency of selection since upon MTXselection, MTX-resistant cells can be frequently obtained that have noor poor selectable marker expression. Instead, loss of function of theRFC, the primary MTX transporter may become a frequent mechanism of MTXresistance. This has been shown to be due to the frequent emergence ofinactivating mutations in the RFC gene or severe loss of RFC geneexpression.

d) The selection system according to the present invention does not usea cytotoxic drug and/or mutagenic compound such as MTX in the DHFRsystem or MSX in the GS system that can alter the genotype of therecipient cells as well as of the target gene of interest. Rather, theFR selection utilizes the principle of deprivation of a vitamin from thegrowth medium.

Accordingly, in one aspect the present invention thus relates to aeukaryotic expression vector comprising a first polynucleotide encodinga functional membrane-bound folate receptor (i.e. the selectable markergene) and a second polynucleotide encoding a product of interest.

A functional membrane-bound folate receptor according to the presentinvention is particularly defined as a functional membrane-boundreceptor capable of unidirectional import or uptake of a folate into aeukaryotic cell.

A folate according to the present invention can either be an oxidizedfolate (i.e. folic acid) or a reduced folate. In general, a folate maybe useful within the present invention as long as such folate will becapable of being taken up into a eukaryotic cell by the functionalmembrane-bound folate receptor. A preferred example of an oxidizedfolate is folic acid. Preferred examples of reduced folates are5-methyl-tetrahydrofolic acid, 5-formyl-tetrahydrofolic,10-formyl-tetrahydrofolic acid and 5,10-methylene-tetrahydrofolic acid.

In a preferred embodiment, the expression vector of the presentinvention is capable of expressing both the functional membrane-boundfolate receptor and the product of interest in a eukaryotic cell.

The product of interest encoded by the second polynucleotide can be anybiological product capable of being produced by transcription,translation or any other event of expression of the genetic informationencoded by the second polynucleotide. In this respect, the product willbe an expression product. For example, in a preferred embodiment, such aproduct is selected from the group consisting of a polypeptide, a RNA,and a DNA. A “polypeptide” refers to a molecule comprising a polymer ofamino acids linked together by peptide bond(s). The term “polypeptide”includes polypeptides of any length, which may be called “protein” incase of a larger molecule (comprising for example more than about 50amino acids), or “peptide” in case of a smaller molecule (comprising forexample 2-49 amino acids). The product can be a pharmaceutically ortherapeutically active compound, or a research tool to be utilized inassays and the like. In a particularly preferred embodiment, the productis a polypeptide, preferably a pharmaceutically or therapeuticallyactive polypeptide, or a research tool to be utilized in diagnostic orother assays and the like. In a most preferred embodiment thepolypeptide is an immunoglobulin molecule or antibody, or a fragment (inparticular a functional fragment) thereof, for example a chimeric, or apartly or totally humanized antibody. Such an antibody can be adiagnostic antibody, or a pharmaceutically or therapeutically activeantibody. Typically, the product of interest will be heterologous to theeukaryotic host cell used for expression, which means that the host celldoes not naturally or endogenously produce the product of interestbefore transfection. Rather, in order to achieve production orexpression of the product of interest a polynucleotide encoding theproduct of interest has to be introduced into the eukaryotic host cell,in particular by transfection with an expression vector according to thepresent invention.

A vector according to the present invention can be present in linearform or, preferably, in circular form, e.g. a plasmid.

Vectors used for expression of polynucleotides of interest usuallycontain transcriptional control elements suitable to drive transcriptionsuch as e.g. promoters, enhancers, polyadenylation signals,transcription pausing or termination signals. If the desired product isa protein, suitable translational control elements are usually includedin the vector, such as e.g. 5′ untranslated regions leading to 5′ capstructures suitable for recruiting ribosomes and stop codons toterminate the translation process. In particular, both thepolynucleotide serving as the selectable marker gene as well as thepolynucleotide encoding for the product of interest will be transcribedunder the control of transcription elements present in appropriatepromoters. The resultant transcripts of both the selectable marker geneand that of the product of interest harbor functional translationelements that facilitate substantial levels of protein expression (i.e.translation).

Accordingly, a preferred embodiment relates to an expression vectoraccording to the present invention wherein the first polynucleotide andthe second polynucleotide are under the control of distincttranscription promoters. In general, a promoter capable of promotingexpression, in particular transcription, of the first and/or secondpolynucleotide in a eukaryotic will be suitable. In a preferredembodiment, the distinct transcription promoters are the same. Inanother preferred embodiment the distinct transcription promoters aredifferent. Preferably, the transcription promoters are selected from thegroup consisting of an SV40 promoter, a CMV promoter, an EF1 alphapromoter, a RSV promoter, a BROAD3 promoter, a murine rosa 26 promoter,a pCEFL promoter and a β-actin promoter. In a preferred embodimentthereof the promoter controlling the transcription of the firstpolynucleotide and/or second polynucleotide is CMV promoter or, mostlypreferred, an SV40 promoter. In a particularly preferred embodiment thepromoter controlling the transcription of the first polynucleotide is aSV40 promoter.

In another preferred embodiment of an expression vector of the presentinvention the first polynucleotide and the second polynucleotide areunder the control of a common transcription promoter. Preferably, suchtranscription promoter is selected from the group consisting of an SV40promoter, a CMV promoter, a RSV promoter, a BROAD3 promoter, a murinerosa 26 promoter, a pCEFL promoter and a β-actin promoter. In a furtherpreferred embodiment thereof the common transcription promoter is anSV40 promoter. A further preferred embodiment of the expression vectorhaving such a common transcription promoter comprises an IRES elementfunctionally located between the first polynucleotide and the secondpolynucleotide.

The membrane bound folate receptor as introduced into the eukaryotichost cell by means of an expression vector utilized according to thepresent invention can be derived from any species as long as it will befunctional within the present invention, i.e. compatible with theeukaryotic cell utilized. Preferably, a folate receptor derived from amammalian species will be used, for a example derived from a rodent, or,mostly preferred, a human folate receptor. In general, the folatereceptor introduced into the eukaryotic host cell and utilized asselection marker can be homologous or heterologous to an endogenousfolate receptor of the host cell. If it is homologous it will be derivedfrom the same species as the host cell, and may, for example, beidentical to an endogenous folate receptor of the host cell. If it isheterologous it will be derived from another species than the host cell,and may thus be different from an endogenous folate receptor of the hostcell. Typically, the introduced folate receptor utilized as theselection marker will be heterologous to the host cell. For example ahuman-derived folate receptor may be used as selection marker for arodent host cell, e.g. a CHO cell.

Preferably, the functional membrane-bound folate receptor encoded by thefirst polynucleotide of an expression vector of the present invention isselected from the group consisting of the folate receptor alpha (FRα),the folate receptor beta (FRβ), and a functional mutant thereof. Afunctional mutant comprises a derivative of a folate receptor which isfunctional in a physiological manner, i.e. capable of being uptaken bythe eukaryotic cell and contributing to the cell's viability via thecell's folate metabolism. For example, a mutant form of the folatereceptor will comprise one or more amino acid mutation(s), like asubstitution, deletion and/or addition, as well as a chemicalderivative, where a chemical moiety, like a polymer, for example apolyethylene glycol structure (PEG), is attached to the folate receptor.Preferably, the folate receptor encoded by the first polynucleotide is ahuman folate receptor alpha (hFRα), a human folate receptor beta (hFRβ),or a functional mutant thereof. Most preferred is a human folatereceptor alpha (hFRα), preferably having the following amino acidsequence (SEQ ID NO 1, 1-letter code, shown in direction from N-terminusto C-terminus):

MAQRMTTQLLLLLVWVAVVGEAQTRIAWARTELLNVCMNAKHHKEKPGPEDKLHEQCRPWRKNACCSTNTSQEAHKDVSYLYRFNWNHCGEMAPACKRHFIQDTCLYECSPNLGPWIQQVDQSWRKERVLNVPLCKEDCEQWWEDCRTSYTCKSNWHKGWNWTSGFNKCAVGAACQPFHFYFPTPTVLCNEIWTHSYKVSNYSRGSGRCIQMWFDPAQGNPNEEVARFYAAAMSGAGPWAAWPFLLSLAL MLLWLLS

Another preferred embodiment relates to a human folate receptor beta(hFRβ) having the following amino acid sequence (SEQ ID NO 2, 1-lettercode, shown in direction from N-terminus to C-terminus):

MVWKWMPLLLLLVCVATMCSAQDRTDLLNVCMDAKHHKTKPGPEDKLHDQCSPWKKNACCTASTSQELHKDTSRLYNFNWDHCGKMEPACKRHFIQDTCLYECSPNLGPWIQQVNQTWRKERFLDVPLCKEDCQRWWEDCHTSHTCKSNWHRGWDWTSGVNKCPAGALCRTFESYFPTPAALCEGLWSHSYKVSNYSRGSGRCIQMWFDSAQGNPNEEVARFYAAAMHVNAGEMLHGTGGLLLSLALMLQ LWLLG

In an alternative, the present invention relates to a folate receptorwhich in its natural environment is not membrane-bound. Such anon-membrane bound receptor can be mutated in order to becomemembrane-bound, for example by providing a fusion protein between thenon membrane-bound folate receptor and a transmembrane region of anotherpolypeptide. Likewise, other mutant forms are possible which would bereadily available for a person skilled in the art. Preferred examples inthis respect would be based on the soluble folate receptor gamma (FRγ),preferably the human soluble folate receptor gamma (FRγ). In a mostpreferred embodiment thereof, the human soluble folate receptor gamma(FRγ) would have the following amino acid sequence (SEQ ID NO 3,1-letter code, shown in direction from N-terminus to C-terminus):

MDMAWQMMQL LLLALVTAAG SAQPRSARAR TDLLNVCMNAKHHKTQPSPE DELYGQCSPW KKNACCTAST SQELHKDTSRLYNFNWDHCG KMEPTCKRHF IQDSCLYECS PNLGPWIRQVNQSWRKERIL NVPLCKEDCE RWWEDCRTSY TCKSNWHKGWNWTSGINECP AGALCSTFES YFPTPAALCE GLWSHSFKVSNYSRGSGRCI QMWFDSAQGN PNEEVAKFYA AAMNAGAPSR  GIIDSwhich then may be mutated or otherwise genetically altered orderivatized to form a functional membrane-bound folate receptor capableof folate uptake within the context of the present invention.

In a further aspect, the expression vector according to the presentinvention can additionally comprise one or more furtherpolynucleotide(s) encoding one or more additional selection marker(s).Accordingly, in a preferred embodiment co-selection utilizing the folatesystem of the present invention together with one or more differentselection system(s) (e.g. neo/G418) can be applied to provide optimalperformance.

In another aspect, the present invention relates to a eukaryotic cellfor which cellular viability is dependent on folate uptake, and intowhich eukaryotic cell a first polynucleotide located on an expressionvector and encoding a functional membrane-bound folate receptor and asecond polynucleotide located on an expression vector and encoding theproduct of interest have been stably introduced, wherein the firstpolynucleotide and the second polynucleotide are located on the sameexpression vector or on separate an expression vectors. In a preferredembodiment thereof, the functional membrane-bound folate receptor andthe product of interest are expressed by the eukaryotic cell.

In addition to the functional membrane-bound folate receptor introducedinto the cell line via an expression vector, the eukaryotic cellaccording to the present invention can comprise at least one endogenousfunctional unidirectional functional folate transport system, inparticular one or more endogenous functional membrane-bound folatereceptor(s). It is an advantage of the present invention that the methodof selection as described herein below can be utilized even in thepresence of such endogenous unidirectional functional folate transportsystem, i.e. where such endogenous system is retained. Accordingly, afurther preferred embodiment relates to the eukaryotic cell of thepresent invention relates, comprising at least one endogenousunidirectional functional folate transport system, wherein suchendogenous unidirectional functional folate transport system preferablycomprises at least one endogenous functional membrane-bound folatereceptor. In a preferred embodiment thereof, the endogenous functionalmembrane-bound folate receptor is selected from the group consisting ofthe folate receptor alpha (FRα) and the folate receptor beta (FRβ).

Another preferred embodiment relates to a eukaryotic cell according tothe present invention, wherein the endogenous unidirectional functionalfolate transport system, for example comprising at least e.g. oneendogenous functional membrane-bound folate receptor, is lacking fullactivity, i.e. is attenuated. Such attenuation can be provided forexample by any type of mutagenesis of the endogenous folate transportsystem in question, e.g. the endogenous functional membrane-bound folatereceptor, for example by point mutation, gene disruption, and the like.The attenuation can be a partial or complete. In the latter case theeukaryotic cell according to the present invention does not comprise anendogenous functional unidirectional functional folate transport system,e.g. an endogenous functional membrane-bound folate receptor.Accordingly, a preferred embodiment the present invention relates tosuch a eukaryotic cell wherein an expression vector of the presentinvention has been stably introduced, and which cell is lacking fullactivity of at least one endogenous functional membrane-bound folatereceptor.

With respect to the expression vector introduced into said host cell anyexpression vector of the present invention, including its preferredembodiments, as described herein, can be utilized. In a preferredembodiment of the eukaryotic cell of the present invention the firstpolynucleotide encoding a functional membrane-bound folate receptor andthe second polynucleotide encoding the product of interest are locatedon the same expression vector. Preferably, such expression vector is andexpression vector according to the present invention, i.e. as describedherein.

The eukaryotic cell according to the present invention is, preferably,selected from the group consisting of a mammalian cell, an insect cell,a plant cell and a fungi cell. With respect to fungi cells and plantcells, which usually are prototrophic for folates (i.e. such cells canautonomously synthesize their own folates necessary for their cellularviability, i.e. cellular growth and proliferation). The presentinvention encompasses in particular such fungi and plant cells which maybecome auxotrophic for folates. This may be for example due to geneticmanipulation, i.e. cells are now unable to synthesize sufficient amountsof folates necessary for their cellular viability. For example, thecapacity of such fungi or plant cells to endogenously biosynthesizefolates, e.g. via an appropriate metabolic pathway, will be inactivated,e.g. by gene disruption or gene silencing of appropriate target genes,or inhibition of key enzymes, etc.

In a preferred embodiment thereof the eukaryotic cell is a mammaliancell. Preferably, such mammalian cell is selected from the groupconsisting of a rodent cell, a human cell and a monkey cell.Particularly preferred is a rodent cell, which preferably is selectedfrom the group consisting of a CHO cell, a BHK cell, a NSO cell, a mouse3T3 fibroblast cell, and a SP2/0 cell. A most particularly preferredrodent cell is a CHO cell. Also preferred is a human cell, which,preferably, is selected from the group consisting of a HEK293 cell, aMCF-7 cell, a PerC6 cell, and a HeLa cell. Further preferred is monkeycell, which, preferably, is selected from the group consisting of aCOS-1, a COS-7 cell and a Vero cell.

In another embodiment the present invention relates to a process forproduction of a eukaryotic cell according to the present invention, saidprocess comprising providing an eukaryotic cell for which cellularviability is dependent upon folate uptake, and introducing a firstpolynucleotide located on an expression vector and encoding thefunctional membrane-bound folate receptor and a second polynucleotidelocated on an expression vector and encoding the product of interest,wherein the first polynucleotide and the second polynucleotide arelocated on the same expression vector or on separate an expressionvectors. In a preferred embodiment, the first polynucleotide and thesecond polynucleotide are located on the same expression vector which,in a most preferred embodiment, is an expression vector according to thepresent invention, i.e. as disclosed herein.

A yet other aspect of the present invention relates to a method forselection of a eukaryotic cell capable of stably expressing a product ofinterest encoded by an expression vector which has been introduced intothe cell, comprising

(i) providing a plurality of eukaryotic cells for which cellularviability is dependent upon folate uptake, and into which cells a firstpolynucleotide located on an expression vector and encoding a functionalmembrane-bound folate receptor and a second polynucleotide located on anexpression vector and encoding the product of interest have beenintroduced, wherein the first polynucleotide and the secondpolynucleotide are located on the same expression vector or on separateexpression vectors,

(ii) culturing said plurality of eukaryotic cells in a cell culturemedium having a limiting concentration of a folate, thereby obtaining aeukaryotic cell wherein stable expression of the product of interest isachieved. In principle, such folate can be an oxidized folate or areduced folate. Preferred is an oxidized folate, which in particular isfolic acid.

With respect to the limiting amount of a folate the suitableconcentration in the medium can be determined by a person skilled in theart in accordance with the requirements of the host cell and thestringency of the selection condition to be applied. In case that folicacid is used as the folate, for example with a CHO host cell, a suitableconcentration of folic acid in the cell culture medium for a stringentselection process would be about 100 nM or lower, preferably about 30 nMor lower, or about 10 nM or lower. For example, a suitable concentrationof folic acid can have any value in the range of 0.001 nM-100 nM,preferably in the range of 0.01 nM-100 nM, more preferably in the rangeof 0.1 nM-100 nM or in the range of 1 nM-100 nM. Likewise preferred isthe range of 0.001 nM-30 nM, the range of 0.01 nM-30 nM, the range of0.1 nM-30 nM, the range of 1 nM-30 nM, or the range of 3 nM-10 nM. Forexample, the folic acid concentration in the cell culture mediumsuitable for selection can be 1 nM, 3 nM, 10 nM or 30 nM.

In case that a reduced folate like leucovorin will be used in theselection process the concentration thereof again can be determined by aperson skilled in the art in accordance with the requirements of thehost cell and the stringency of the selection condition to be applied.Such concentration of leucoverin in the cell culture medium can be forexample be in the range of 0.2 nM-2 nM for a stringent selectionprocess.

In a further embodiment thereof, the method for selection furthercomprises identifying and isolating a eukaryotic cell wherein stableexpression of the product of interest is achieved.

In a most preferred embodiment of the method for selection, theplurality of eukaryotic cells is composed of eukaryotic cells accordingto the present invention, i.e. as disclosed herein.

Further preferred embodiments of this aspect of the represent inventionare described herein, in particular with respect to the eukaryotic celland the expression vector.

Another embodiment of the present invention relates to a process forproduction of a product of interest, comprising

(i) performing a method of selection according to the present invention,i.e. as disclosed herein,

(ii) and isolating the product of interest from said cell culture mediumor from said cell.

Again, preferred embodiments of this aspect of the represent inventionare described herein, in particular with respect to the eukaryotic celland the expression vector.

The product of interest, for example a polypeptide, produced inaccordance with the invention may be recovered, further purified andisolated by methods known in the art. For example, the polypeptide maybe recovered from the nutrient medium by conventional proceduresincluding, but not limited to, centrifugation, filtration,ultra-filtration, extraction or precipitation. Purification may beperformed by a variety of procedures known in the art including, but notlimited to, chromatography (e.g. ion exchange, affinity, hydrophobic,chromatofocusing, and size exclusion), electrophoretic procedures (e.g.,preparative isoelectric focusing), differential solubility (e.g.ammonium sulfate precipitation) or extraction.

A yet further aspect of the present invention relates to the use of afunctional membrane-bound folate receptor as a selection marker forselection of a eukaryotic cell, for which eukaryotic cell cellularviability is dependent on the uptake of folate, and which eukaryoticcell being capable of stably expressing a product of interest. Within apreferred embodiment of such use, the folate receptor is selected fromthe group consisting of the folate receptor alpha (FRα), the folatereceptor beta (FRβ), and a functional mutant thereof. Preferably, thefolate receptors utilized within this aspect of the present inventionare the respective human folate receptors, the human folate acidreceptor alpha (FRα) being preferred. Further preferred embodiments ofthis aspect of the represent invention are described herein, inparticular with respect to the eukaryotic cell and the expressionvector.

The full contents of the texts and documents as mentioned herein areincorporated herein by reference.

The following examples serve to illustrate the present invention withoutin any way limiting the scope thereof. In particular, the examplesrelate to preferred embodiments of the present invention.

EXAMPLES

In general, the materials mentioned herein, such as reagents, arefamiliar to the skilled person, commercially available and can be usedin accordance with the manufacturer's instructions.

Example 1 High-Level Expression of a Recombinant Antibody Utilizing theFolate-Receptor Based Selection System Example 1.1 Expression Vectors

A plasmid vector (i.e. the test vector), suitable for expression ineukaryotic cells, in particular CHO cells, harboring both (i) anexpression cassette which comprises a polynucleotide encoding the heavyand light chains of a secreted recombinant human antibody of IgG1 type,and (ii) an distinct expression cassette which comprises apolynucleotide encoding a human folic acid receptor alpha (hFRα) asselectable marker gene, is constructed to explore the efficiency ofselection of hFRalpha (hFRα)-transfected cells under limitingconcentrations of a folate, i.e. folic acid, in the culture medium.Expression of the human folic acid receptor alpha (hFRα) is undercontrol of an SV40 promoter and a standard (SV40) polyadenylationsignal. Expression of the recombinant antibody is under control of a CMVpromoter and a standard (SV40) polyadenylation signal. As a control(i.e. the control vector), a similar expression vector is used, encodingthe same antibody, and lacking the hFRalpha (hFRα) expression cassette,but containing a neomycin phosphotransferase gene as a selectablemarker.

Example 1.2 Cells and Growth Conditions

Chinese hamster ovary cells derived from strain CHO-K1 are maintainedunder suspension culture conditions in suitable chemically definedgrowth medium containing 2.3 μM (microM) folic acid.

For analysis of folic acid dependency of cell survival, a folic acidstarvation experiment is done using folic acid concentrations rangingfrom 2300 nM to 0.1 nM. Cells are cultivated in such medium and cellviability is analyzed to quantify the percentage of surviving cells.Table 1 summarizes the results obtained with the CHO-K1 cell linementioned above.

TABLE 1 Survival of CHO cells at different folic acid concentrations: FAConcentration Precentage of [nM] Survival 0.1 2.08 1 2.45 3 2 10 2.7 306.8 100 55.5 300 88.6 1000 100.8 2300 100

These results indicate that for this specific host cell line folic acidconcentrations below 100 nM, preferably below 30 nM should be suitableto generate significant selection pressure for folic acid receptor basedselection of stably transfected cells.

Example 1.3 Transfection and Selection

Cells are transfected by electroporation either with the test vectorcontaining a hFRalpha (hFRα) expression cassette or the control vectorlacking the hFRalpha hFRα. The transfectant cells are subsequently grownunder suspension culture conditions in 125 ml shake flasks in mediumsupplemented with an appropriate concentration of glutamine and 2.3 μM(microM) folic acid. Forty eight hours after transfection, cells aretransferred to a medium containing a limited amount of folic acid,namely 10 nM or 1 nM folic acid to initiate the selection processaccording to the invention in 24-well plates for samples transfectedwith the test vector. In addition, cells transfected with the controlplasmid are selected by adding a selection agent, namely 0.8 mg/mL G418,to a medium containing 2.3 μM (microM) folic acid (i.e. control 1) orcultured in the absence of any selection (i.e. control 2). Cells whichhave successfully recovered from this selection scheme are transferredto 6-well plates and further expanded in shake flasks for analysis ofantibody production levels.

Example 1.4 Analysis of Antibody Production

From the transfectant and folic acid deprived cell populations,overconfluent batch cultures in shake flasks are prepared to analyzeantibody expression levels. Cells are seeded at 2×10⁵ cells/mL in mediumcontaining 2.3 μM (microM) folic acid and incubated under suspensionculture (i.e. shaker) conditions. At day 14, the supernatant of the cellculture is harvested and analyzed for antibody levels using a protein-AHPLC methodology, i.e. an affinity-type of purification. IgG moleculesspecifically bind to the column, mainly via their Fc-part, while otherproteins pass the column without interacting with the matrix. Under lowpH conditions, captured IgG proteins are eluted from the column,quantified via UV absorption measurement, and, where necessary, furtherpurified and isolated.

Example 1.5 Results

The aim of this approach is to provide a proof of concept that a folategene, in particular the hFRalpha (hFRα) gene, can serve as a selectablemarker under conditions of folate deprivation thereby selecting cellsthat co-overexpress a product of interest., e.g. a monoclonal antibody.As a control, a vector containing a neomycin-resistance gene as aselectable marker is also used. After transfection, cells are subjectedto a stringent selection by abruptly reducing the folic acidconcentrations in the medium from 2.3 μM (microM) to 10 nM or 1 nM.Cells transfected with a plasmid harboring the folic acid receptorreadily recover under conditions of folate deprivation and can befurther expanded in the selective medium. In contrast, in the case ofthe control vector, the concentration of folic acid remains unchanged,but either a selection pressure with G418 or no selection pressure atall are applied. The selected cell populations are then analyzed forantibody production using overgrown (i.e. overconfluent) suspension(i.e. shake) flask cultures in medium containing 2.3 μM (microM) folicacid. The concentration of antibody in the culture medium is thendetermined at day 14. As shown in Table 1 below, cells transfected withthe plasmid containing the folic acid receptor and selected by reducingfolic acid availability, overexpress the recombinant antibody. Theamount of antibody produced by these transfectant cell populations ishigher compared to the cells transfected with the control vector andselected with G418. As a further control, when no selection pressure isapplied, cells providing no antibody production are obtained. These dataprovide the proof of concept that the current folic acid receptor geneapproach can be readily applied as a stand-alone dominant metabolicselectable marker to rapidly establish cells overexpressing arecombinant product of interest.

TABLE 2 (C_(Folic acid:) concentration of folic acid in the mediumduring selection; C_(G418): concentration of G418 in the medium duringselection; C _(mAb): concentration of the secreted antibody in themedium of overgrown cultures) C_(Folic acid) C_(G418) C_(mAb) Vector(nM) (mg/ml) (mg/l) Test vector 10 none 25 (hFRalpha 1 none 24 hFRα))Control vector 2300 0.8 8 (Neo) 2300 none 0

Example 2 Recombinant Antibody Production Levels Increase as a Functionof the Decrease in Folic Acid Concentration in the Growth Medium Example2.1 Expression Vector

A plasmid vector (i.e. the test vector) as described in Example 1.1above is provided.

Example 2.2 Cells and Growth Conditions

Chinese hamster ovary cells derived from strain CHO-K1 are maintainedunder monolayer culture conditions in chemically defined growth mediumRPMI-1640 containing 2.3 μM (microM) folic acid. The cells are lackingRFC transporter activity, as disclosed elsewhere (Assaraf, Y. G. andSchimke, R. T. (1987) Proc. Natl. Acad. Sci. USA 84, 7154-7158; Rothem,L., et al., Mol. Pharmacol. 68: 616-624). Such a RFC-deficient cell isused to avoid a potential by-pass of the folic acid starvation by thisfurther carrier system in this example.

Example 2.3 Transfection and Selection

RFC-deficient C15 cells are transfected with the test vector byelectroporation. Forty eight hours after transfection, cells arepropagated in folic acid-free medium supplemented with 30 nM folic acidin order to promote the expression of both the selectable marker as wellas the recombinant antibody and subsequently subjected for dilutioncloning. Cells are diluted to a final density of 5 cells/ml and seededat 100 μl/well in 96-well plates (i.e. 0.5 cells/well). Clones are thenmaintained in a medium containing 0.25 nM folic acid and 500 μg/ml G418.The co-selection utilizing the folate system of the present inventiontogether with an additional selection system (i.e. neo/G418) is appliedto provide optimal performance of the selection process. Clones with thehighest levels of antibody production are then grown under low folicacid concentrations (i.e. 1200 pM, 600 pM, and 60 pM) to further supportand establish antibody overexpression. This is corroborated by furtheranalysis of antibody expression in the various clones.

Example 2.4 Analysis of Antibody Production

The analysis of antibody production is performed in principle asoutlined in Example 1.4 above. The concentration of the secretedantibody is monitored using an ELISA assay as follows: Maxisorpmicroplates are coated with an anti-human IgG. Following blocking with abuffer containing bovine serum albumin (BSA) and several washes,multiple dilutions of the secreted antibody samples are added. Then, aperoxidase-conjugated second antibody consisting of goat anti-humanIgG-peroxidase is added. Finally, a colorimetric peroxidase substrate isadded following which the resultant dye concentration is determined ineach well spectrophotometrically and then compared to standardconcentrations of known IgG concentrations.

2.5 Results

As depicted in Table 2 below, the levels of recombinant antibodyproduction correlate with the stringency of folic acid deprivation.Hence, antibody production levels increase as the concentration of folicacid concentration is decreased in the medium. These results furthercorroborate the proof of concept that the hFRalpha (hFRα) gene is anefficient selectable marker that can be used for the overexpression ofrecombinant proteins under conditions of folate deprivation.

TABLE 3 (C_(Folic acid:) concentration of folic acid in the medium; C_(mAb:) concentration of the secreted antibody in the medium)C_(Folic acid) (pM) C_(mAb) (μg/L) 60 216 ± 30 600 138 ± 15 1200 19 ± 8

1. A eukaryotic expression vector for recombinant expression of aproduct of interest in a mammalian cell comprising a firstpolynucleotide encoding a functional membrane-bound folate receptorserving as selectable marker in a cell culture medium having a limitingconcentration of a folate and a second polynucleotide encoding a productof interest to be recombinantly expressed, wherein the product ofinterest is a pharmaceutically active, therapeutically active, or adiagnostic polypeptide.
 2. The expression vector according to claim 1,wherein the first polynucleotide and the second polynucleotide are underthe control of distinct transcription promoters.
 3. The expressionvector according to claim 2, wherein the transcription promoters are thesame.
 4. The expression vector according to claim 2, wherein thetranscription promoters are different.
 5. The expression vectoraccording to claim 2, wherein the promoter controlling the transcriptionof the first polynucleotide is an SV40 promoter.
 6. The expressionvector according to claim 1, wherein the first polynucleotide and thesecond polynucleotide are under the control of a common transcriptionpromoter.
 7. The expression vector according to claim 6, wherein thecommon transcription promoter is an SV40 promoter.
 8. The expressionvector according to claim 6, wherein said vector comprises an IRESelement functionally located between the first polynucleotide and thesecond polynucleotide.
 9. The expression vector according to claim 1,wherein the functional membrane-bound folate receptor encoded by thefirst polynucleotide is selected from the group consisting of a folatereceptor alpha (FRα), a folate receptor beta (FRβ), and a functionalmutant thereof.
 10. The expression vector according to claim 9, whereinthe functional membrane-bound folate receptor encoded by the firstpolynucleotide is a human folate receptor alpha (hFRα).
 11. A mammaliancell for recombinantly producing a product of interest wherein thecellular viability of said cell is dependent on folate uptake, and intowhich mammalian cell a first polynucleotide located on an expressionvector and encoding a functional membrane-bound folate receptor servingas selectable marker in a cell culture medium having a limitingconcentration of a folate and a second polynucleotide located on anexpression vector and encoding a product of interest have been stablyintroduced, wherein the first polynucleotide and the secondpolynucleotide are located on the same expression vector or on separatean expression vectors and wherein the product of interest is apharmaceutically active, therapeutically active, or diagnosticpolypeptide.
 12. The mammalian cell according to claim 11, wherein saidcell is lacking full activity of at least one endogenous functionalmembrane-bound folate receptor.
 13. The mammalian cell according toclaim 11, wherein said first polynucleotide encoding a functionalmembrane-bound folate receptor and said second polynucleotide encoding aproduct of interest are located on the same expression vector.
 14. Themammalian cell according to claim 11, wherein the expression vector is aeukaryotic expression vector for recombinant expression of a product ofinterest in a mammalian cell comprising a first polynucleotide encodinga functional membrane-bound folate receptor serving as selectable markerin a cell culture medium having a limiting concentration of a folate anda second polynucleotide encoding a product of interest to berecombinantly expressed, wherein the product of interest is apharmaceutically or therapeutically active or a diagnostic polypeptide.15. The mammalian cell according to claim 11, wherein said mammaliancell is a rodent cell.
 16. The mammalian cell according to claim 15,wherein said rodent cell is a CHO cell.
 17. A process for production ofa mammalian cell according to claim 13, said process comprisingproviding a mammalian cell for which cellular viability is dependentupon folate uptake, and introducing a first polynucleotide located on anexpression vector and encoding the functional membrane-bound folatereceptor serving as selectable marker in a cell culture medium having alimiting concentration of a folate and a second polynucleotide locatedon an expression vector and encoding the product of interest, whereinthe first polynucleotide and the second polynucleotide are located onthe same expression vector or on separate an expression vectors.
 18. Theprocess according to claim 17, wherein the first polynucleotide and thesecond polynucleotide are located on the same expression vector.
 19. Theprocess according to claim 18, wherein the expression vector is aeukaryotic expression vector for recombinant expression of a product ofinterest in a mammalian cell comprising a first polynucleotide encodinga functional membrane-bound folate receptor serving as selectable markerin a cell culture medium having a limiting concentration of a folate anda second polynucleotide encoding a product of interest to berecombinantly expressed, wherein the product of interest is apharmaceutically or therapeutically active or a diagnostic polypeptides.20. A method for selection of a eukaryotic cell capable of stablyexpressing a product of interest encoded by an expression vector whichhas been introduced into the cell, comprising (i) providing a pluralityof eukaryotic cells for which cellular viability is dependent uponfolate uptake, and into which cells a first polynucleotide located on anexpression vector and encoding a functional membrane-bound folatereceptor and a second polynucleotide located on an expression vector andencoding the product of interest have been introduced, wherein the firstpolynucleotide and the second polynucleotide are located on the sameexpression vector or on separate expression vectors, (ii) culturing saidplurality of eukaryotic cells in a cell culture medium having a limitingconcentration of a folate, thereby obtaining a eukaryotic cell whereinstable expression of the product of interest is achieved.
 21. The methodaccording to claim 20, further comprising identifying and isolating aeukaryotic cell wherein stable expression of the product of interest isachieved.
 22. The method according to claim 20, wherein the plurality ofeukaryotic cells is composed of eukaryotic cells as defined in claim 11.23. A process for production of a product of interest, comprising (i)performing a method of selection according to claim 11, and (ii)isolating the product of interest from said cell culture medium or fromsaid cell.
 24. A method for selection of a eukaryotic cell capable ofstably expressing a product of interest comprising the use of afunctional membrane bound folate receptor as a section marker, whereinsaid eukaryotic cell cellular viability is dependent on the uptake offolate.
 25. The method according to claim 24, wherein the folatereceptor is selected from the group consisting of the folate receptoralpha (FRα), the folate receptor beta (FRβ), and a functional mutantthereof.
 26. The method according to claim 25, wherein the folatereceptor is the human folate receptor alpha (hFRα).